a) Field of the Invention
The present invention relates to a real image type viewfinder optical system which is preferable for use with compact cameras and thin cameras in particular such as photographic cameras and video cameras.
b) Description of the Prior Art
Under the present circumstances where image reflecting optical members such as porro prisms which are effectively usable for making display members such as visual field frames more legible and employing lens elements having smaller diameters can be manufactured by monolithic molding of plastic materials, lens shutter cameras adopt more and more real image type viewfinder optical systems such as the keplerian viewfinder optical system disclosed by Japanese Patent Kokai Publication No. Sho 63-226,616 in place of virtual image type viewfinder optical systems such as Albada type and inverted Galilean type viewfinder optical systems.
The conventional real image type viewfinder optical system has a fundamental composition illustrated in FIG. 1, wherein a reference numeral 1 represents an objective, a reference numeral 2 designates an image inverting optical member configured as a porro prism and a reference numeral 3 denotes an eyepiece. That is to say, the conventional real image type viewfinder optical system has a fundamental composition wherein the objective and the eyepiece are disposed before and after respectively the porro prism, thereby inevitably having large thickness or being limited in shortening of a length thereof in a direction along an optical axis thereof.
Further, the real image type viewfinder optical system disclosed by Japanese Patent Kokai Publication No. Sho 63-226,616 is configured so that the objective forms an image in an image inverting member 2 and a visual field frame is located on the imaging surface. However, this conventional example required to divide, into two parts, a porro prism which can be molded or shaped as an integral member, thereby being composed of a large number of parts and undesirable from a viewpoint of the shortening of a length thereof in the direction along an optical axis thereof.
As a viewfinder optical system which has solved the problem posed by the conventional example described above, there is known the viewfinder optical system disclosed by Japanese Patent Kokai Publication No. Hei 4-19,705 in which an eyepiece and a final reflecting surface of an image inverting optical member are integrated with each other so as to form a prism. This conventional example has a composition illustrated in FIG. 2. The viewfinder optical system exhibits a certain effect for shortening a length thereof in the direction along an optical axis thereof, but does not permit shortening its length sufficiently due to a fact that it allows an objective disposed therein to protrude on the object side.
Further, known as another viewfinder optical system configured for compactness is the one disclosed by Japanese Patent Kokai Publication No. Hei 6-300,966 obtained by the applicant of the present invention. This viewfinder optical system uses an objective which is integrated with an image iverting member so as to form a prism 4 having a first reflecting surface, thereby exhibiting effects for shortening a length of the optical system in the direction along an optical axis Lc thereof and forming a thin porro prism. However, this viewfinder optical system cannot be configured thinner since it requires a certain minimum length as measured from a surface of emergence 4a of a porro prism 4 to a final surface 5a of an eyepiece in the direction along an optical axisl Lc thereof as shown in FIG. 3.